While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration densities and operating speeds in LSI devices, DUV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The lithography techniques which are considered promising next to the ArF lithography include electron beam (EB) lithography, F2 lithography, extreme ultraviolet (EUV) lithography, and x-ray lithography. In these techniques, exposure must be done in vacuum or reduced pressure, which allows the sulfonic acid generated during exposure to volatilize, failing to form a satisfactory pattern profile. The sulfonic acid volatilized can damage the exposure system. In the EB and EUV lithography, it is desired to make a resist material more sensitive in order to reduce the burden on the exposure system.
Under the circumstances, development efforts were made to introduce photoacid generator units into a resin for the purposes of improving resolution, reducing PED size change, and suppressing dissolution of acid generator additive in immersion liquid (typically water) during the ArF immersion lithography. Patent Document 1 (JP-A 2008-133448) describes a polymer comprising units of triphenylsulfonium 1-(difluorosulfomethyl)-2,2,2-trifluoroethyl methacrylate (or triphenylsulfonium 1,1,3,3,3-pentafluoro-2-methacryloyloxypropane-1-sulfonate). Patent Document 2 (JP-A 2009-263487) describes that a polymer comprising the above-mentioned units and polymerization units having a phenolic hydroxyl group has a higher sensitivity. Patent Document 3 (JP-A 2010-077377) discloses 4-fluorophenyldiphenylsulfonium 1,1-difluoro-1-sulfoethyl methacrylate, a polymer comprising the same as polymerization units, and a resist material.
According to the inventors' research work, the sulfonium salt having a triphenylsulfonium cation has a fully high sensitivity to ArF and KrF exposures, but an insufficient sensitivity to EUV exposure. Even when the salt is combined with a specific polymerization unit as in Patent Document 2, an improvement in sensitivity is still desired. The problem has not been overcome by Patent Document 3 because the sulfonium salt itself is low soluble in polymerization solvents and the resulting polymer is less soluble in resist solvents.
Like the prior art KrF and ArF resists, the EUV resist also suffers from a problem of line edge roughness (LER), which is desired to be overcome.